The U.S. auto industry is on a roll by most accounts, with industry volume sizzling and profits riding high on a SUV and pickup-truck boom. There is a weak spot, however.

Retail demand for bread-and-butter passenger cars, such as the
Toyota
TM 0.38%
Camry and Chevrolet Cruze, has softened amid lower gasoline prices. This is leading high-volume auto makers to send more of these types of cars to rental car fleets, a practice that keeps production lines humming but can dent resale values and clip margins.

About 21% of the compact cars and 20% of the midsize cars sold during the first quarter ended up in daily rental fleets, up from about 17% in the same period a year earlier, according to the most recently available U.S. registration data. For subcompact cars, the daily rental mix was 28% for the period, up from 24% in the prior year.

Registration data, compiled by IHS Automotive, was provided to The Wall Street Journal by industry sources. IHS declined to confirm the numbers, saying its data is proprietary.

The data presents a troubling caution flag for a U.S. auto industry enjoying the strongest demand in more than a decade. The push toward rental-car fleets indicates a key portion of the market is being artificially supported.

Fleets have historically offered auto makers a dumping ground for less popular models. Detroit auto makers were often criticized for using these sales as a way to support the production of cars that real buyers didn’t want to own, and now—as the practice is reviving—import brands are also using the strategy.

The pressure to sell cars to daily rental companies could intensify as new plants in North America add more production capacity over the next few years.

Among the most reliant on sales to rental fleets in the first quarter:
General Motors Co.
GM 0.69%
,
Fiat Chrysler Automobiles
FCAU 0.84%
NV,
Volkswagen AG
, Nissan Motor Co. and Hyundai Motor Co. The Chevy Cruze had the highest volume of cars going to rental car fleets during the quarter, amounting to about 45% of the compact’s total sales, followed by the Nissan Altima, Toyota Camry and Chrysler 200.

About one-quarter of sales for the popular Camry, the best-selling passenger car in the U.S., were to rentals in the first quarter, up from 14% a year ago. Roughly 39% of Chrysler 200 sedan sales went to rental fleets. For the Nissan Altima, the rental-car mix was 27%.

Auto makers, including Toyota, GM and Fiat Chrysler, say fleet sales are seasonal and reflect demand on the part of rental-car firms, which tend to buy small and midsize cars in high volumes. Company officials say the percentage going to rental fleets should drop as the year goes on.

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Susan Lombardo, head of vehicle acquisition in the U.S. for Enterprise Holdings Inc., said the rise in cars going to rental-car lots is one way auto makers are dealing with a sudden shift in tastes. “This is more of a stop-gap measure.”

Ms. Keller said auto makers are starting to play a larger role in piquing rental-car company interest and that can be costly. The need to do so, she said, is “indicative of the creep of excess supply in this industry.”

First-quarter sales through March increased 5.6%, and momentum increased in April and May. With gasoline prices relatively low, light-truck sales rose 11.7% while passenger cars fell 0.2%. Trucks and SUVs represented 53.6% of industry sales, compared with 50.9% in 2014 and 48.5% in 2013.

Passenger cars are high-volume staples for the full-line auto makers. While SUVs and pickups do well when gas is cheap and deliver higher margins, having a lineup of modern family sedans and small cars is important when fuel prices snap back to higher levels.

“Auto makers are at a crossroads on what to do,” Edmunds.com analyst Jessica Caldwell said.

Auto makers want to keep plants running at close to full steam, and that has them looking for buyers to take other passenger-car models that aren’t as popular as they were when high gas prices had retail buyers wanting good fuel economy.
Hertz Global Holdings Inc.,
HTZ 2.42%
aiming to replace its aging fleet, has been a big buyer over the last several months.

Maryann Keller, a longtime auto analyst and industry adviser, said auto makers are being more restrained in their approach and are willing to cut production if car stockpiles get too high. “It’s not the free-for-all it once was,” she said. “They’re not shoving cars down their throats.”